This invention relates generally to heat transfer machines and, more particularly, to an improved linkage for a platen type press useful for applying decals onto articles such as clothing.
Heat transfer machines are widely utilized to apply premanufactured decals or "transfers" onto numerous articles and substrates such as sweatshirts, t-shirts, papers and plastics. Such machines are typically clamshell type presses with upper and lower platens which are brought into registry and compressed for a preselected period of time to apply the transfer to the article which is positioned on the lower platen. The upper and/or lower platen is typically heated to facilitate bonding of the transfer with the substrate. After the transfer has been applied, the press is opened by upward pivoting of the upper platen to permit removal of the clothing with the transfer applied thereto. Another clothing article and transfer may then be placed on the lower platen and the upper platen lowered to apply the transfer to the clothing.
When operating heat transfer machines of the type described above, it is important that adequate pressure be applied to the platens to ensure proper adherence of the transfer to the clothing. It is also desirable to be able to lock the upper platen into compression against the lower platen to provide a uniform pressure during the time period required to apply the transfer to the substrate. Linkages such as a type known as a four-bar linkage are commonly used to achieve the desired pressure and locking action. The upper platen is mounted to one bar of the linkage and another bar serves as a lever arm which is manipulated by the machine operator. The force applied by the operator on the lever arm is amplified by the linkage so that a greater force is applied to the upper platen than could otherwise be exerted by the operator. In addition, when the upper platen is fully compressed against the lower platen, a slight overclosure of the linkage provides a locking action which maintains the platens in compression. The locking action of the linkage may be readily overcome by the operator lifting upward on the lever arm to open the press after the transfer has been applied to the article in the press.
Because of the repetitive nature of the transfer application process, it is important that the machine operator be able to remain stationed in front of the machine while moving the lever arm between the fully opened and closed position. This requirement restricts the range of movement of the lever arm and the force that may be effectively applied with linkages of the type described above. The force which may be supplied by such linkages is further restricted by the need for the upper platen to have a large travel path so that the transfer article may be placed by the operator on the lower platen with less risk of the operator accidentally burning his hands on the hot undersurface of the upper platen. In order to achieve the desired opening of the upper platen, a substantial portion of the lever arm range of motion must be dedicated to pivoting movement of the upper platen. Only a small portion of the lever arm travel then remains for compression of the platens.
With the advent of new types of transfers and processes such as embossing which require large pressures for effective application and the desire for larger platen sizes, a need has developed for platen presses capable of delivering greater compression forces. Some platen presses are now available with a pneumatic closing mechanism which supplies increased pressure by using a cylinder or diaphragm to compress the platens. The pneumatic closing device, however, may be hazardous if improperly designed or used. Such devices also significantly increase the cost of the press and a source of compressed air is necessary to operate the closing device.
Other attempts to supply the necessary platen travel and compression force have focused on sideways swinging of the upper platen after compression between the platens has been released. Such an arrangement often requires manipulation of two handles and is more difficult for the operator to manage in comparison to the simple one-motion closing and pressurization provided by the linkage previously described. A need thus remains for a linkage which is operable in a single motion to provide a large range of platen travel and the large compression forces required by current transfer materials and techniques.